Apparatus and method for heat treating a fluid product

ABSTRACT

An apparatus and a method for heat treating a fluid product by injection of steam therein in order to provide a sterilization, pasteurization and/or a homogenization of the product. The apparatus includes a mixing chamber having a steam injection inlet, a product inlet, and an outlet for the treated product, a rotatable shaft disposed within the mixing chamber, a plurality of discs each comprising at least one product passage opening, the discs being disposed substantially co-axially on the shaft, and a motor for rotating the shaft with the discs. The invention also relates to a shaft to be used for homogenizing and mixing in an apparatus or a process for heating by steam injection a fluid product.

TECHNICAL FIELD

The present invention relates to an apparatus for treating a fluid suchas for sterilising and/or pasteurising and/or homogenising fluidproducts. More particularly, it relates to an apparatus for sterilising,pasteurising and/or homogenising a fluid product by injection of steaminto the fluid product.

BACKGROUND ART

Direct steam injection is e.g. used in the food industry forpasteurising or sterilising of milk or cereal products. Various types ofapparatus for pasteurising or sterilising fluid food products byinjection of steam are known. The present invention relates to a methodand apparatus wherein steam is injected into a product stream.

Whereas direct steam injection does not raise particular problems forlow viscosity fluids such as milk, whey and cream, the pasteurisation ofe.g. infant cereal based food, with far higher viscosity, e.g.comprising 20% starch, is very difficult to control in a homogeneousmixing. An inhomogeneous mixing will mean that temperature variesthrough the product mass and consequently the product is nothomogeneously heat treated. In order to safeguard sufficiently heattreatment of the product i.e. to make sure that all parts of the productmass are heated to a high enough temperature, such as is crucial inpasteurising and sterilising processes, additional steam will need to beinjected. This additional steam consumption is expensive andundesirable.

Homogenisation and mixing may be improved by passing the product througha venturi shaped nozzle in order to increase turbulence and velocity ofthe product, however, this may not provide a sufficient enough mixing.Alternatively, a mechanical mixing of the product and the steam may benecessary.

Mixing of the product and steam may be obtained by passing the productthrough a static mixer. However, substantial wear of the static mixer islikely to result from the passage of highly viscous products therethrough. Such static mixer will therefore have to frequently bereplaced, which increases production costs. Alternatively, a screw maybe used for mixing and for building up a pressurised product. However,the product creates a large momentum on the screw when rotated, andhence considerable motor capacity is required for the mixing.

SUMMARY OF THE INVENTION

The present invention overcomes the above mentioned difficulties byutilising a rotatable shaft along which a plurality of perforated discscomprising product passage openings, are co-axially disposed.Accordingly, the invention provides an improved apparatus and processwhich accommodates a good mixing and thus a homogeneous heating togetherwith a surprisingly high homogenisation. Among other things, theinvention has the following advantages:

Lowered steam consumption by ensuring a sufficiently homogeneous mixing.

Improved mixing and homogenising by providing a rotatable shaft alongwhich a plurality of perforated discs comprising product passageopenings are coaxially disposed and by passing the product through theopenings while rotating the discs,

Shock effect due to flowing product hitting the discs and the passage ofthe product through the opening resulting in turbulent flow and mixing,and homogenisation.

Cutting through the product mass by rotating the discs to increase thehomogenisation of the product.

Reducing motor capacity requirement due to a reduced momentum from theproduct surrounding the shaft and the disc,

Optimising heat distribution by providing an initial homogenising of theproduct where after the steam is injected and the product and steam aremixing together for a condensation of the steam and a heat transfer tothe product.

Accordingly, in a first aspect the present invention relates to anapparatus for heat treating a fluid product by injecting steam into thefluid product, the apparatus comprising:

a mixing chamber comprising a steam injection inlet, a product inlet,and an outlet for the treated product,

a rotatable shaft disposed within the mixing chamber,

a plurality of discs each comprising at least one product passageopening, the discs being disposed substantially co-axially on the shaft,and

means for rotating the shaft with the discs.

It is preferred that the steam be injected at sonic velocity. This willincrease the turbulence in the steam and a good mixing is obtained.Sonic velocity of steam can be obtained by letting the fluid passthrough a venturi valve while having upstream a pressure which isapproximately twice that downstream. The steam velocity after the valvewill then be about 450 m/sec.

Advantageously, at least on the downstream end of the mixing chamberonly a small gap is left between the discs and the mixing chamber walls.This results in substantially the entire product mass passing throughthe product passage opening in the discs and substantive homogenising ofthe fluid product and mixing of steam and product. Conveniently, atleast the downstream part of the mixing chamber is substantiallycylindrical and the rotatable shaft is substantially co-axially fittedwithin the mixing chamber.

One or more steam injectors may be positioned along the shaft forinjection of steam, via the steam injection inlet, into product passingthrough the mixing chamber. The rotating shaft with discs will, whilethe product is pumped through the mixing chamber, cut the product streamand act as a shock generator such as discussed above. The shaft anddiscs will, contrary to a screw, not in any substantial way add to theadvancing movement of the product through the mixing chamber. Theproduct will be pumped into the chamber and advanced by further productbeing supplied. The shaft may be provided with replaceable discs or theshaft and discs may be made integral. The shaft may be rotated by anyconventional motor.

Depending on the required throughput, heating, mixing andhomogenisation, the number of discs and the openings therein may bevaried. The more discs provided along the products path from the productinlet to the outlet for the treated product the more homogenising andmixing will be provided. Although the number of discs can be freelychosen, for practical purposes the number of discs can be from 2 to 20,preferably 5 to 15. The diameter of the product passage openings ispreferably in the range of from about 2 to 10 mm, more preferably 6 mm.This will provide a velocity into the product passage openings ofbetween about 40 and 200 m/s, preferably 100 m/s.

In the present context a fluid product is understood to be a solution orsuspension, e.g. aqueous, having a viscosity of the order of about 1000to 100,000 mPa.s.

The fluid product is pumped into the mixing chamber through the productinlet. It is preferred that the fluid product is first passed through atthe product passage opening(s) in at least one and preferably 3 rotatingdiscs before steam is injected into the product. This gives an initiallyhomogeneous slurry which is then pressed forward and subjected to steaminjection heating, mixing, and homogenising.

The homogenising effect is obtained by the shearing of the productpassing through the openings of the discs. In the particular case ofmilk-containing products, a good homogenising effect is obtained by thearrangement through shearing of the fatty globules.

The steam injection nozzle is connected to a feed pipe feeding steamunder pressure, preferably between about 10 and 15 bar. The productinjection nozzle is connected to a feed pipe feeding the fluid productto be treated. The outlet from the apparatus is by a back-pressure valveconnected to a holding pipe. The back-pressure valve regulates thepressure through the outlet and creates a back pressure in the mixingchamber to maintain the temperature in the mixing chamber. Poor controlof the back pressure results in loss of steam which gives an uneconomicproduction. The back-pressure valve may advantageously be of themembrane type. Advantageously, two pack-pressure valves may be used e.g.CPM or APC valves from Tetra-Laval.

In another aspect, the invention relates to a method for heat treating afluid product by injecting steam into the fluid product, the methodcomprising the steps of

supplying a fluid product into a mixing chamber through a product inlet,

injecting steam into the fluid product through a steam injection inletin the mixing chamber, in which mixing chamber being disposed arotatable shaft and a plurality of discs each comprising at least oneproduct passage opening, the discs being disposed substantiallyco-axially on the shaft, while

passing at least part of the fluid product through the product passageopenings while rotating the shaft with the discs, and

discharging the treated fluid product through an outlet in the mixingchamber.

In a further aspect, the invention relates to a shaft to be used forhomogenising and mixing in an apparatus or a process for heating bysteam injection a fluid product, the shaft comprising a plurality ofdiscs each comprising at least one product passage opening, the discsbeing disposed substantially co-axially on the shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail with reference tothe accompanying drawings given by way of example showing an embodimentof the invention and in which

FIG. 1 is a schematic illustration of the principle elements of theapparatus according to the invention, and

FIG. 2 and FIG. 3 are schematic illustrations of the shaft with discsfor such an apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the apparatus 10 according to the invention. The apparatus10 comprises a mixing chamber 11 defined by walls 21 wherein a steaminjection inlet 13, a product inlet 12, and an outlet 16 are formed. Arotatable shaft 14 is disposed within the mixing chamber 11 and mountedin bearings 18 within an intermediate support 22. It is connected to anoutput rotary shaft 23 of a motor 17 which drives the rotation of theshaft 14. This motor may e.g. be a 1.1 Kw, 1400 t/min motor. A sealing24 is provided to prevent fluid product from escaping the mixing chamber11. This sealing 24 is cooled by means of water supplied via a waterinlet 25.

The shaft 14 is provided with a plurality of discs 15. Each disc 15comprises at least one product passage opening. The discs are disposedsubstantially co-axially on this shaft. FIG. 2 shows a perspective viewof the discs 15 mounted on the shaft 14. These discs include productpassage openings 26. FIG. 3 shows a cross-sectional view of the shaft 14with a disc 15 fitted within the chamber 11.

The flow through the outlet 16 is regulated by means of a back-pressurevalve 27. To use the apparatus a steam inlet 13 is connected to steaminjector and a feed pipe for supplying steam under pressure, and theproduct inlet is connected to a feed pipe and a pump for supplying thefluid product to be treated under pressure. The outlet is preferablyconnected to a holding pipe by back-pressure valve 27.

EXAMPLE

The following example illustrates the substantial and surprising steamreduction obtained with the apparatus according to the invention whencompared with processing of a similar fluid product in a steam injectionnozzle i.e. a nozzle wherein both streams of steam and fluid product areinjected into a mixing chamber counter-currently without any furthermechanical mixing (S&P injection). The apparatus according to theinvention comprises a shaft with 10 discs. The diameter of the discs was44 mm and each disc had 6 product passage openings of 8 mm diameter. Theexample shows test results of heat treatment of fluid products withvarying solid matter content. The products were heated to 140° C.:

    ______________________________________            Solid matter                        Steam consumption                                     (Kg/Hour)    Product Ts soup %   S&P injection                                     Invention    ______________________________________    A       60-62%      320          195    B       56-58%      330          195    C       58-60%      330          195    D       58-60%      335          200    E       60%         350          215    F       56-58%      355          200    ______________________________________

Thus, the apparatus of the invention utilizes only about 55 to 65% asmuch steam as the conventional device. This results in a savings of 40to 45% in the amount of steam needed. Depending upon the specificconfiguration of the device of the invention and the solids content ofthe products to be treated, steam savings of between about 25 and 50%are easily achieved.

What is claimed is:
 1. A method for heat treating a fluid food productby injecting steam into the fluid food product, said method comprisingthe steps of:supplying a fluid food product into a mixing chamberthrough a product inlet, injecting a sufficient amount of steam from asteam supply into said fluid food product through a steam injectioninlet in said mixing chamber to heat treat the fluid food product, inwhich mixing chamber is disposed a rotatable shaft and a plurality ofdiscs, each comprising at least one product passage opening co-axiallyand discs being disposed substantially on said shaft, contacting saidsteam and said fluid food product within said mixing chamber by passingat least part of the fluid food product through said product passageopenings while rotating said shaft and discs to heat treat said productand form a homogeneous mixture of the heat treated fluid food productand the steam within said chamber, and discharging the homogeneousmixture through an outlet in the mixing chamber.
 2. Method according toclaim 1 which further comprises injecting said steam into the fluid foodproduct at a substantially sonic velocity.
 3. Method according to claim1 wherein said fluid food product is passed through one of the productpassage openings in at least one of said discs prior to contacting saidsteam.
 4. Method according to claim 1 wherein said steam is injected ata pressure of from about 10-15 bar.
 5. Method according to claim 1 whichfurther comprises controlling a flow of the heated product through saidoutlet with a back pressure valve.
 6. Method according to claim 1 whichfurther comprises selecting the fluid food product from the groupconsisting of milk, whey and cereal.
 7. Method according to claim 1wherein said fluid food product contains greater than about 20% solids.8. Method according to claim 7 wherein the fluid food product isselected to be infant cereal.